Implantable medical devices (IMDs), including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with an external device (ED) via a radio-frequency telemetry link. One such external device is an external programmer used to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data that may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.
External programmers are commonly configured to communicate with an IMD over an inductive link. Coil antennas in the external programmer and the IMD are inductively coupled so that data can be transmitted by modulating a carrier waveform which corresponds to the resonant frequency of the two coupled coils. An inductive link is a short-range communications channel requiring that the coil antenna of the external device be in close proximity to the IMD, typically within a few inches. Other types of telemetry systems may utilize far-field radio-frequency (RF) electromagnetic radiation to enable communications between an IMD and an ED over a wireless medium. Such long-range RF telemetry allows the IMD to communicate with an ED, such as an external programmer or remote monitor, without the need for close proximity. In either the home or the clinic, however, there are external sources of RF energy which may interfere with communication between the ED and IMD.
It is also common in clinical settings for there to be multiple implantable and/or external devices are present in an area so that communication over the wireless medium is possible between the multiple devices. Access to the medium among the multiple devices must be controlled in this situation in order for a communications session between any pair of devices to be established. It would also be desirable for there to be the possibility of multiple communications sessions between different devices occurring concurrently.